The invention may be conveniently used to prepare for the slow rotation or "inching" of a grinding mill and will be described with reference to such a mill.
A charge or load in a grinding mill may become frozen as a result of a stoppage or shut down of the mill. It is common practice to inch the mill, that is, to rotate the mill very slowly, to slowly tilt the mill charge and permit it to cascade or tumble. This prevents a lumped or frozen charge from being rotated to a top position within the mill and having it instantaneously fall to the bottom with disastrous results. When the mill is inched, the rotation or inching can be stopped if the charge does not cascade before a certain point is reached.
It is convenient to inch the mill electricially, that is, to apply a low frequency alternating current to the stators of the synchronous motors with the rotor field windings excited normally from a DC (direct current) supply. The low frequency might be, for example, of the order of one cycle per second. With the clutches closed or locked, this will provide a slow rotation of the mill provided that the motors are in precise angular coincidence. If they are not in precise angular coincidence, a destructive hammering of any associated gearing will result.
The low frequency alternating current is often provided from a DC source through contactors which close and open in a predetermined sequence to provide a simulated or artificially synthesized alternating current which increases and decreases in discrete steps as the contactors operate. Other forms for an alternating current power source might, of course, be acceptable. However, the step-type alternating supply obtained by using contactors and a DC source is relatively inexpensive and adequate for the purpose of inching. One such source of alternating current could provide inching power for shared use between several grinding mills at one location as inching is not required very frequently.
The placing of synchronous motors in angular coincidence has sometimes been referred to as "synchronizing" the motors for load sharing. However, because the "synchronizing" of synchronous motors has another meaning in the art, this description will refer to the angular coincidence or angular alinement of synchronous motors for load sharing.
When electric inching is required, it is essential that the two (or more) synchronous motors which drive the mill be in angular coincidence with one another before load is applied. In other words, it is very desirable that the synchronous motors, at each instant, when energized from the same electrical supply, should have exactly the same load angle, that is, have the same angular coincidence so that load is equally shared. Thus, the angular coincidence will ensure load sharing between the motors under steady state and transient conditions. Otherwise, if the rotors are coupled to the common load with a small difference in angular coincidence with respect to the rotating field, there will not be load sharing. A very small difference in load angle measured at the clutches may represent a large difference in motor load, and result in destructive rattling of the mill gear train during the inching operation.
In the past where more than one synchronous motor was used on a mill drive, these were permanently coupled to the gear train in the correct electrical and mechanical relationship during installation. The coupling means was not readily adjustable during the operation of the mill and required many hours of down time to make a mechanical correction. It was found that during the inching operation relatively small errors in machine load angle would cause destructive rattling of the mill gear train because the machines were continuously transferring load torque between one another. It will be apparent that if the machines are to be connected to the gear train or mill through friction clutches, which offer no predetermined angular relationship, it will be impossible to achieve satisfactory performance.
The present invention provides for achieving the necessary precision of angular coincidence or alinement of the synchronous motors prior to inching. The rotor field DC excitation is applied to the plurality of stationary motors. The inching contactors are closed at a predetermined point in their sequence, and the DC voltage supply which provides a DC voltage source for the contactors is ramped up or increased to a predetermined level. As the DC voltage is increased, the resulting stator current develops an alining torque unless the excited field poles of each motor are exactly alined with the stator magnetizing flux. Thus the rotors rotate until precise angular coincidence or alinement is achieved before the clutches are closed. The clutches are then closed and mechanically locked and the commutating sequence of the contactors initiated to start inching. Because the rotors were positioned before the clutches were closed, and because the inching started from this common rotor position, the motors share the load with precision.